1. Field of the Invention
This invention relates to the disposal of CHF3.
2. Description of Related Art
Fluoroform (CHF3, HFC-23) is a by-product of the reaction of HF with trichloromethane to form chlorodifluoromethane (CHF2Cl, HCFC-22), which is the primary source of perfluoroolefin, such as tetrafluoroethylene (TFE). The fluoroform by-product constitutes less than about 3 wt % of the HCFC-22 formed, but because annual production of HCFC-22 is large worldwide, the amount of fluoroform by-product made amounts to several millions of pounds per year. The fluoroform by-product either has to be used or has to be subject to disposal.
U.S. Pat. No. 3,009,966 discloses that fluoroform is thermally inert (col. 1, I. 13-14), but nevertheless finds a use for the fluoroform as a source of TFE and hexafluoropropylene (HFP) by pyrolysis of the fluoroform at temperatures of 700-1090xc2x0 C., with temperatures of 1000xc2x0 C. and higher being required to obtain conversions of at least 50% for the fluoroform at contact (pyrolysis times) of 0.1-0.12 sec. (Tables 1 and 2). The higher yields of HFP are accompanied by increasing amounts of perfluoroisobutylene (PFIB), which is toxic. Even at lower pyrolysis temperatures, the yields of PFIB can be quite high. U.S. Pat. No. 6,025,532 discloses the pyrolysis of fluoroform to a mixture of HF, TFE and HFP at a temperature of at least 700xc2x0 C., but actually at 1000xc2x0 C. at a contact time of 32 milliseconds (Examples), followed by contacting the mixture with a fluorination catalyst to obtain HFC-125 (CF3CHF2) and/or HFC-227ea (CF3CHFCF3). The high temperature required for pyrolyzing fluoroform at short contact times has limited the use of fluoroform by-product, whereby excess fluoroform has been available, which to avoid venting to the atmosphere has been disposed of by incineration.
Several references disclose the use of fluoroform in an auxiliary pyrolysis role. WO 96/29296 discloses the co-pyrolysis of HCFC-22 with fluoroalkane to form primarily large molecule fluoroalkanes. In particular, the reference discloses this reaction being carried out wherein the fluoroalkane co-reactant is fluoroform and the pyrolysis temperature is 700xc2x0 C. and the contact time is 10 seconds, to obtain 100% conversion of the HCFC-22, with the result being a 60% yield of pentafluoroethane (Example 1). The disadvantage of this process, besides the extraordinarily long contact time, is that 40% of the yield is apparently not useful product. It is impractical to attempt to dispose of HFC-23 by consuming it in a process which produces such a high yield of by-product which itself needs disposal. Example 1 also reports that perfluoropropene is formed, without quantifying its amount, which is characteristic of reporting trace amounts detectable in the gas phase chromatography analysis used. The Examples of this reference are conducted with an aqueous alkaline wash of the pyrolysis reaction mixture to eliminate the HCl co-produced. The washing could also limit the ultimate reaction product to saturated HFC compounds. In the Examples the reactor is quartz. Quartz reacts with hydrogen fluoride, a probable intermediate in the pyrolysis reaction of HFC-23 and HCFC-22. The elements of hydrogen fluoride are part of the process according to the present invention and its consumption in side reactions, as with quartz, would lead to a reduction in the production of saturated hydrofluorocarbons.
Another reference disclosing the auxiliary use of fluoroform in a pyrolysis reaction is U.S. patent application Ser. No. 09/878,540, filed Jun. 11, 2001 (U.S. patent application Publication Ser. No. 2002/0032356-A), which discloses the pyrolysis of HCFC-22 in a gold-lined reactor to direct the synthesis reaction to the formation of the fluoroolefins TFE and HFP, without forming significant amounts of PFIB. The Examples disclose the co-pyrolysis of HCFC-22 and HCFC-124 (CF3CHFCl) to favor the formation of HFP over TFE. The possibility of fluoroform (CHF3) being present with the HCFC-22 is also disclosed as a recycle gas in the reactor system, the fluoroform thereby being the major component fed to the reactor, indicating that the fluoroform is acting as an inert carrier in the pyrolysis process, as would be expected from the relatively low pyrolysis temperatures and short contact times disclosed. Such use of fluoroform is not an effective way to dispose of fluoroform.
The problem remains of finding an economically acceptable use for the fluoroform by-product so that it does not have to be incinerated.
The present invention solves this problem by consuming fluoroform (HFC-23) to economically produce useful product by co-pyrolyzing the fluoroform with chlorodifluoromethane (HCFC-22) at a temperature in the range of about 625-800xc2x0 C., preferably about 690-775xc2x0 C. and contact time of less than two seconds, and obtaining as a result thereof a product mixture of useful saturated and unsaturated compounds, i.e. at least three compounds selected from the group consisting of pentafluoroethane (CF3CHF2, HFC-125), heptafluoropropane (CF3CHFCF3, HFC-227ea), TFE, and HFP, respectively. The process can be carried out by feeding the mixture of reactants (HCFC-22 and HFC-23) through a reaction zone, the surface of which is metal, preferably gold, to minimize the formation of perfluoroisobutylene by-product in the pyrolysis reaction.
Unexpectedly, the HFC-23 pyrolyzes at the relatively low temperature of the co-pyrolysis reaction in short contact times to produce a high yield, e.g. at least 80%, of the above-mentioned useful products and little to no detectable PFIB. Apparently, the presence of the HCFC-22 in the pyrolysis reaction reduces the reaction (decomposition) temperature of the HFC-23 so that the latter is consumed in the pyrolysis reaction. Typically at least 4 parts by weight of HFC-23 is consumed for each 100 parts by weight of HCFC-22 such that the amount of HFC-23 consumed is greater than the amount produced as by product during the manufacture of HCFC-22.
The function of the fluoroform in the present invention is to increase the amount of useful saturated two- and three-carbon atom compounds, CF3CHF2 (HFC-125) and CF3CHFCF3 (HFC-227ea), along with production of TFE and HFP.
The pyrolysis reaction in the present invention is carried out by continuous feeding of the co-reactants to a pyrolysis reactor and continuously withdrawing the resultant mixture of reaction products and unreacted reactants from the reactor. Pyrolysis reactors generally comprise three zones: a) a preheat zone, in which reactants are brought close to the reaction temperature; b) a reaction zone, in which reactants reach reaction temperature and are at least partially pyrolyzed, and products and any by-products form; c) a quench zone, in which the stream exiting the reaction zone is cooled to stop the pyrolysis reaction, preferably to 500xc2x0 C. or lower, to reduce coking or polymerization downstream of the reaction zone. xe2x80x9cCokexe2x80x9d is solid carbonaceous material that accumulates in, and on the surface of, the reactor. The resulting fouling is undesirable because it interferes with heat transfer and fluid flow. Quenching may be accomplished by interior cooling or exterior cooling, or both.
The reactor can be tubular, wherein the pyrolysis reaction occurs in the interior of the tube, and the tube can have a variety of cross-sectional shapes, such as circular, oval (elliptical) or polygonal, said shapes being of the interior or of the exterior surfaces of the tube, or both. The tubular reactor will typically have an inner diameter in the case of circular cross-section of at least about 0.125 in (0.32 cm), preferably about 0.125 in (0.32 cm) to about 3 meters, more preferably about 0.5 in (1.27 cm) to about 2 m, and most preferably about 0.7 in (1.8 cm) to about 1 m. The ratio of volume to surface area of a tubular reactor of unit length and of interior radius R can be determined by dividing the surface area A (A=2 xcfx80R) into the volume V (V=xcfx80xc2x7R2). If R is in centimeters, V/A=(R/2) cm3/cm2. In this way it can be stated that the volume to surface ratio is at least about 0.08 cm3/cm2, preferably about 0.08 cm3/cm2 to about 75 cm3/cm2, more preferably about 0.32 cm3/cm2 to about 50 cm3/cm2, and most preferably about 0.64 cm3/cm2 to about 25 cm3/cm2.
The reactor is made of metal, such as nickel or nickel alloy. The exposed surface of the reaction zone in particular is of a metal that resists corrosion at the pyrolysis temperatures of reaction of HCFC-22 and HFC-23. Nickel or nickel alloys such as Inconel(copyright) or Hastelloy(copyright) are preferred, Inconel(copyright) is more preferred. Most preferred is gold, because gold is more resistant to the corrosive action of hydrogen halides and the formation of coke than are nickel-based materials. Gold has the further advantage of suppressing PFIB formation. Whereas the process of this invention with a nickel or nickel alloy reactor generates less than about 5% PFIB based on the combined weight of TFE, HFP, HFC-125 and HFC-227ea, in a gold reactor less than about 2% PFIB is formed on the same basis. xe2x80x9cExposed surfacexe2x80x9d refers to the surface that is exposed to the reactants and/or reaction products in the reaction zone. Apart from using gold as the material of the surface of the reaction zone and optionally of the exposed surface of the quench zone, the reactor can be of conventional design.
The gold on the interior surface of the reaction zone must be supported by a heat-resistant, thermally conductive material of construction, such as a metal which has a melting temperature of at least about 1100xc2x0 C. and which gives structural integrity to the reactor. Inconel(copyright) and Hastelloy(copyright) are nickel alloys suitable for use as supporting materials for the gold lining of the reactors (see for example U.S. Pat. No. 5,516,947). Other thermally conductive supporting materials can be used. Thermal conductivity enables the reactor to be externally heated to provide the interior temperature necessary for the pyrolysis reaction. It is desirable that the supporting material be metallurgically bonded to the gold lining for the best heat transfer. By a metallurgical bond is meant a bond in which atoms of the metals in the supporting material and the gold lining interdiffuse, that is, diffuse among each other about the bonded interface. U.S. patent application Publication Ser. No. 2001/0046610 (Nov. 29, 2001) discloses a method for making a gold-lined tube in which the gold lining is metallurgically bonded to the supporting material.
Normally a plurality of the tubular reactors will be positioned within a shell, and a heating medium will be flowed between the interior wall of the shell and the exterior walls of the tubular reactors bundled therein to provide the heating for the pyrolysis reaction. Alternatively, the shell can be exteriorly heated or fired by means such as electrical means to provide the interior heating. The combination of the shell and the tubular reactors positioned therein forms the pyrolysis furnace. Alternatively, the reactor may consist of a single reaction vessel, where the required heat for the reaction is other means such as hot inert gas mixed with the reactants. Use of hot inert gas to supply some or all of the heat needed for the reaction reduces or eliminates the heat that must be supplied through the reactor wall. Supplying heat through the reactor wall requires that the wall be hotter than the contents of the reaction space. This condition can lead to undesirable reactions and to decomposition of reactants, intermediates, or products at the wall. The greater the reactor cross-section, the higher wall temperatures must be to supply the necessary heat. Therefore, heating by means of hot inert gas becomes more attractive as the reactor cross-section increases. Examples of hot inert gases which can be used include helium and tetrafluoromethane.
Preferably, the residence time (contact time) in the reaction zone is less than about 1.5 seconds, and more preferably the residence time is about 0.01 to about 1 seconds and even more preferably, from about 0.05 to about 0.8 seconds. Residence time is determined by dividing the net volume of the reaction zone by the volume feed rate in seconds of the gaseous feed to the reactor at reaction temperature and pressure.
The gas temperature within the reaction zone is considered to be the pyrolysis reaction temperature and is measured using a thermocouple in the gas phase in the reaction zone. The reaction zone is heated to a temperature sufficient for the pyrolysis reaction to occur, preferably within the reaction time of less than 1.5 seconds.
Preferably, the HCFC-22 and HFC-23 are preheated to temperatures approaching but not reaching the temperatures at which their respective pyrolyses begin. Preheating reduces the amount of heat that must be provided in the reaction zone and thereby reduces the temperature difference between the walls of the reaction zone and the gas feed. The closer the wall and the gas temperatures are to the desired reaction temperature, the fewer will be side reactions generating undesirable products and reactor fouling. HCFC-22 and HFC-23 may be mixed and preheated together. The preferred preheating temperature when the two gases are fed together is between about 500xc2x0 C. and 600xc2x0 C., and most preferably between about 550xc2x0 C. and 600xc2x0 C. If the gases are preheated separately the HCFC-22 is preheated to about 300xc2x0 C. to 450xc2x0 C., and the HFC-23 is preheated to about 500xc2x0 C. to 600xc2x0 C.
In another embodiment in which HFC-23 and HCFC-22 are preheated separately, the HFC-23 is preheated to at least about 850xc2x0 C., and the HCFC-22 is preheated to about 300xc2x0 C. to 550xc2x0 C. This embodiment is preferred for adiabatic reaction of HFC-23 and HCFC-22 or to reduce the amount of heat that must be supplied to the reaction by heating the reaction vessel. It takes advantage of the thermal stability of HFC-23 to heat it to less than its decomposition temperature (e.g. conversion of no more than 3%) in the absence of HCFC-22. The heat in the HFC-23 supplies some or all of the heat necessary for the reaction of HFC-23 with HCFC-22 and reduces or eliminates the need for heat to be provided to the reaction vessel. The quantity of heat provided will depend upon the amount of HFC-23 in relation to the amount of HCFC-22.
Depending upon contact times and reaction zone temperatures as well as feed ratios, HFC-23 and HCFC-22 may not be consumed completely in a single pass through the reactor. In continuous processes it is often most efficient to operate at less than 100% conversion so as to maximize production of desired products and minimize undesirable products and fouling. When conversion is less than 100% in the process of this invention, the stream exiting the reactor is treated by conventional methods such as distillation to separate products from unreacted reactants, and the unreacted reactants are mixed with fresh HFC-23 and HCFC-22 to bring the resulting mixture to the desired composition, and the mixture is fed back into the reactor. It may also be desirable to recycle some of the products. For example, CF3CFHCl (HCFC-124), CF2ClCF2H (HCFC-124a), and octafluorocyclobutane ((CF2)4, c318) if formed, can be separated from other products such as HFC-125, HFC-227ea, TFE, and HFP, and added to the reactor feed mixture. Pyrolysis of HCFC-124 and HCFC-124a in the presence of HCFC-22 and HFC-23 contributes to the production of HFP. c318 contributes to TFE production. Through recycling, more HFC-23 can be consumed than is produced in the original manufacture of HCFC-22.
In another embodiment, the flow of the feed through the reactor is partially obstructed to cause back-mixing, i.e. turbulence, and thereby promote mixing of reactants and good heat transfer, further reducing the necessary residence time of the feed in the reactor, e.g. to less than about one-half second. This partial obstruction can be conveniently obtained by using perforated baffles or packing. Increased back-mixing can also be accomplished by increasing the feed rate so as to cause turbulent flow through the reactor.
The volume ratios of HFC-23:HCFC-22 are preferably about 1:10 to 5:1. One preferred ratio is about 2:1 to 5:1, more preferred being about 2:1 to 4:1. Another preferred ratio is no greater than 1:1, such as 1:10 to 1:1. Unreacted HFC-23, is recovered and recycled along with the unconverted HCFC-22. Enough fresh HFC-23 and HCFC-22 are added to this recycle stream to make up for the material converted in the reactor. Preferably the residence time in the reaction zone (contact time), and the relative proportions of HCFC-22 and HFC-23, are such that overall conversion is at least about 10% and yield to useful products is at least about 90%.
HFC-125 finds use as a refrigerant and HFC-227ea finds use as a propellant and fire extinguishant.
The reactor is operated at a temperature, residence time and HFC-23:HCFC-22 ratio such that at least 3 parts of the HFC-23 is converted relative to 100 parts of HCFC-22 converted in order that the amount of HFC-23 consumed is greater than the amount of HFC-23 produced as a by-product during the manufacture of HCFC-22 and which HFC-23 is usually less than about 3 wt %.